A ceramic substrate for mounting a semiconductor device has a metallized pattern to connect with an electrode of the semiconductor device, formed on a face(s) of the ceramic substrate. Further, if the ceramic substrate is used for example as a multilayer substrate or a submount, an electroconductive via for conduction between upper and lower portions of the substrate is formed on the ceramic substrate (hereinafter, the ceramic substrate having the electroconductive via and the metallized pattern is sometimes referred to as a “metallized via-holed ceramic substrate”).
As a method for manufacturing the metallized via-holed ceramic substrate, a co-firing method (simultaneous firing method) and a post-firing method (sequential firing method) are known. In the co-firing method, a metal paste layer is formed on an unfired ceramic substrate precursor called a green sheet, or a metal paste is filled into a through-hole formed in the green sheet, to thereby prepare and fire a metallized via-holed ceramic substrate precursor. In this method, the green sheet and the metal paste are fired simultaneously.
In the post-firing method, a metal paste layer is formed on a sintered substrate obtained by firing a green sheet, or a metal paste is filled into a through-hole formed in the sintered substrate, to thereby prepare and fire a metallized via-holed ceramic substrate precursor. In this method, the green sheet and the metal paste layer are fired sequentially.
Both methods enable formation of the metallized pattern on the ceramic substrate, and the substrate obtained by the methods is mainly used as a substrate for mounting an electronic component. In the co-firing method, however, the green sheet tends to shrink unevenly when fired. If a square green sheet is sintered for example, a central portion on each side thereof shrinks slightly to warp inward, and the substrate deforms into a star shape. Therefore, when many metallized patterns having the same shape and electroconductive vias are formed on one green sheet, the shape of the patterns and the position of the vias inevitably change slightly depending on the location of the patterns. Moreover, in the co-firing method, the metal paste and the green sheet are simultaneously fired at high temperature, and thus, the method has such disadvantages that it requires use of a high-melting-point metal paste of molybdenum, tungsten etc., as the metal paste, and does not allow use of other metals excellent in electroconductivity.
On the other hand, in the post-firing method, the metal paste layer is formed on the sintered substrate, or the metal paste is filled into the through-hole formed in the sintered substrate, and then fired to thereby form the metallized pattern and the electroconductive via. In printing (firing) the metal paste layer, the metal paste layer shrinks in its thickness direction, but hardly shrinks in its planar direction. Thus, there is not a problem that the shape of the pattern changes depending on its location, as is seen in the co-firing method.
However, since the metal paste itself does shrink, shrinkage of the metal paste in the through-hole occurs upon sintering, causing voids in the formed electroconductive via, and making it difficult to form a dense via.
Patent document 1 discloses a method comprising forming, by sputtering, a titanium layer and a copper layer on a ceramic substrate having a through-hole, and thereafter performing copper electroplating, to thereby form a wiring pattern and an electroconductive via. Since the method requires the sputtering step and thus necessitates a manufacturing facility for the sputtering, it does not allow simple manufacturing of the metallized via-holed substrate. Moreover, in filling copper into the through-hole by electroplating in this method, if the diameter of the through-hole or the thickness of the substrate is large, more time will be needed for the filling, causing degradation in the productivity. Additionally, as the filling requires longer time, the thickness of the wiring pattern formed simultaneously and the layer thickness of the resist pattern needed increase, thus making it difficult to attain high precision of the pattern.
When it is desired to reduce the thickness of the wiring pattern in order to form a fine pattern, copper may be deposited thinly only on faces of the through-hole without filling the through-hole completely by electroplating. Thereby as well, the conduction between wirings on both faces of the substrate can be ensured. However, when the substrate is used for example for mounting an LED device and the like, there occur such problems as leakage of a mold resin to the opposite face through the through-hole at a time of forming a lens for the LED device; and rising of a solder to the wiring on the opposite face via the plated face of the through-hole at the time of mounting. These circumstances can be prevented by filling a hole-filling resin paste and the like into the through-hole. However, this causes problems that not only the manufacturing process becomes complicated but also the conductivity of the via degrades.